Keep extension classes in extension package when proguarding

Change-Id: Iefd9d3a22cb5ac4a25fdc762843f3422ae1223e8

appinventor/lib/proguard/proguard.cfg

@@ -6,7 +6,7 @@
 -mergeinterfacesaggressively
 -overloadaggressively
 -useuniqueclassmembernames
--repackageclasses ''
+-keeppackagenames
 
 -keep public class * {
     public protected *;